Analytical Chem istry - DePauw University

Chapter 6 Equilibrium Chemistry223We can divide a redox reaction, such as reaction 6.22, into separatehalf-reactions that show the oxidation and the reduction processes.HCO ( aq) + 2H O() l 2CO ( g) + 2H O ( aq)+ 2e2 2 4 2 2 3Fe( aq) + e Fe ( aq )3 + − 2 ++ −It is important to remember, however, that an oxidation reaction and areduction reaction occur as a pair. We formalize this relationship by identifyingas a reducing agent the reactant undergoing oxidation, becauseit provides the electrons for the reduction half-reaction. Conversely, thereactant undergoing reduction is an oxidizing agent. In reaction 6.22,Fe 3+ is the oxidizing agent and H 2 C 2 O 4 is the reducing agent.The products of a redox reaction also have redox properties. For example,the Fe 2+ in reaction 6.22 can be oxidized to Fe 3+ , while CO 2 can be reducedto H 2 C 2 O 4 . Borrowing some terminology from acid–base chemistry, Fe 2+is the conjugate reducing agent of the oxidizing agent Fe 3+ , and CO 2 is theconjugate oxidizing agent of the reducing agent H 2 C 2 O 4 .Th e r m o d y n a m ic s o f Re d o x Re a c t i o n sUnlike precipitation reactions, acid–base reactions, and complexation reactions,we rarely express the equilibrium position of a redox reaction usingan equilibrium constant. Because a redox reaction involves a transfer ofelectrons from a reducing agent to an oxidizing agent, it is convenient toconsider the reaction’s thermodynamics in terms of the electron.For a reaction in which one mole of a reactant undergoes oxidation orreduction, the net transfer of charge, Q, in coulombs isQ= nFwhere n is the moles of electrons per mole of reactant, and F is Faraday’sconstant (96,485 C/mol). The free energy, ∆G, to move this charge, Q, overa change in potential, E, is∆G = EQThe change in free energy (in kJ/mole) for a redox reaction, therefore, is∆G =−nFE6.23where ∆G has units of kJ/mol. The minus sign in equation 6.23 is the resultof a difference in the conventions for assigning a reaction’s favorable direction.In thermodynamics, a reaction is favored when ∆G is negative, buta redox reaction is favored when E is positive. Substituting equation 6.23into equation 6.3o− nFE =− nFE + RT lnQand dividing by -nF, leads to the well-known Nernst equation